FIG. 5 is a schematic cross-sectional view showing a conventional semiconductor crystal formed on a Si substrate (a base substrate) by crystal growth. MOCVD is applied in a process of crystal growth of the semiconductor crystal. As shown in FIG. 5, a semiconductor crystal (e.g. GaN crystal) grown at a high temperature on a Si substrate (base substrate) by using a conventional technique has a reaction part, dislocations and cracks.
Dislocations and cracks are occurred because of stress which is generated owing to difference of thermal expansion coefficients and difference of lattice constants between different kinds of materials. So when such a crystal growth substrate is used for fabricating each kind of semiconductor devices, the characteristics of the device is deteriorated.
When the base substrate consisting of silicon (Si) except for the grown layer is removed so as to obtain a free-standing substrate (crystal), the substrate cannot have larger area (1 cm2 and bigger) because of dislocations and cracks described above.
At the temperature for crystal growth of an objective semiconductor substrate (semiconductor crystal A), or around 1000° C. to 1150° C., silicon (Si) and gallium nitride (GaN) happen to react (“reaction part” in FIG. 5). As a result, it is not easy to obtain a single crystalline GaN substrate by applying a crystal growing process in a high temperature.
A method of using a silicon thin film, which hardly generates stress described above, alone as a crystal growth substrate in order to obtain monocrystalline GaN substrate is reported. But such a thin film is easily destroyed and it is not easy to handle the thin film directly before carrying out crystal growth. Accordingly, it was difficult to manufacture semiconductor substrates each having a larger area and to obtain a higher yield rate of the semiconductor substrates by employing such conventional methods.